This invention pertains to a cathode-ray tube including a shadow mask having an array of apertures surrounded by a nonapertured border.
In manufacturing a cathode-ray tube for use in color television, a panel assembly is formed which includes a shadow mask having an array of apertures mounted adjacent a substantially rectangular faceplate panel with major and minor axes orthogonal to each other and to a central longitudinal axis passing through the center of the panel. The faceplate panel is made of glass and has a somewhat spherical or domed contour with curvature along both the major and minor axes. The apertures in the shadow mask are typically slit-shaped and arranged in columns that substantially parallel the minor axis of the tube, and the adjacent apertures in each column are separated from each other by bridges or webs in the mask. The apertures are formed typically by etching utilizing photolithographic techniques. The overall shape of the apertured array determines the shape of the picture on the faceplate panel of the tube.
The shadow mask has a nonapertured border comprising an unetched portion which surrounds the etched array of apertures. The border is disposed between the perimeter of the apertured array and a bend, line adjoining a mask skirt. The skirt substantially parallels the central axis and is supported by a shadow-mask frame oriented orthogonally to the central axis. The frame is supported by springs that engage mounting studs that extend inwardly from glass sides of the faceplate panel. The overall shape of the frame, and the bend line of the supported mask, is similar to that of the glass sides of the faceplate panel.
The color cathode-ray tube employs three electron guns for emitting three electron beams which pass through a common deflection yoke, with one beam for each primary phosphor color,.i.e., red, green and blue. The beams are "shadowed" by the apertured mask, so that each beam can strike but one color of a segmented catholuminescent screen of red, green and blue phosphors disposed close to the mask on the inside surface of the faceplate panel. At the point where the electrons from one of the guns impinge on the screen, one of the color phosphors is deposited in a line that approximates the size of the mask aperture. All other parts of the phosphor screen are in the "shadow" of the phosphor mask, as far as this one gun is concerned. Thus, the position and size of the apertures in the shadow mask are important ultimately to achieve good color purity. The rectangular area scanned by the electron beams as they are deflected horizontally and vertically is called the raster. The percentage of the raster which exceeds the perimeter of the apertured array in the shadow mask, along the major axis, is known as overscan. The greater the overscan, the more picture information that is lost due to a greater percentage of the raster being lost by falling outside of the apertured array.
During initial operation of the cathode-ray tube, the shadow mask is heated due to the impingement of the electron beams upon the mask, which absorbs as much as eighty percent of the energy of the beams. The shadow mask is made of relatively thin metal which heats more rapidly than the thicker support frame which serves as a heat sink, thereby resulting in a temperature differential which causes the mask to expand at a greater rate than the frame. Since the shadow mask is peripherally welded to the frame, this more rapid expansion of the shadow mask is resisted by the frame, thereby resulting in mask doming. Such mask doming causes the electron beams, passing therethrough, to misregister with the associated phosphor elements of the screen, resulting in color impurities. This movement can be largely compensated for by the use of temperature-responsive frame supports which cause the mask-frame assembly to move toward the screen in response to the temperature increases in the mask, thereby restoring registration.
Another effect of mask heating, and one that can not be compensated for by temperature-responsive frame supports, is expansion of the mask under certain conditions wherein temperature gradients exist within the mask itself, thereby causing the mask to dome. Such doming causes mask misregistration and variations in misregistration to occur at relatively low overscan, typically three to five percent of total scan. Since more picture information is available at low overscan conditions, it is desirable to be able to minimize doming misregister and variations in doming misregister while operating the cathode-ray tube at relatively low overscan.